Process for the thermal recovery of hydrocarbons from an underground formation



June 10, 1969 w. T. STRICKLAND, JR 3,

PROCESS FOR THE THERMAL RECOVERY OF HYDROCARBONS FROM AN UNDERGROUND FORMATION Filed Dec. 8, 1967 FIG.

FIG, 3

FIG. 4

INVENTOR:

W. T. STRlCKLAND JR HIS ATTORNEY United States Patent U.S. Cl. 166-256 13 Claims ABSTRACT OF THE DISCLOSURE A process for the secondary recovery of hydrocarbons from an underground formation penetrated by an injection and a production well. The combustion of hydrocarbons within the formation adjacent to the injection Well is initiated so as to establish a combustion front therearound. The combustion front is maintained by injecting combustion supporting gas into the formation through the injection well and the front is displaced towards the production well. The combustion front is controlled at a temperature high enough to "permit oxidation of the residual hydrocarbons within the formation at a relatively high rate 'by injecting a quantity of water into the formation through the injection well cocurrently with the injection of the combustion supporting gas. A surface-active material adapted to form a foam is mixed with the gas and water to keep them together in the form of a foam within the formation.

BACKGROUND OF THE INVENTION Field of the invention as a result of which the recoverable quantity of hydrocarbons is increased.

Description of the prior art The ignition of a portion of the hydrocarbons present in a formation may be effected by injecting enough oxidizing gas to cause spontaneous combustion or by raising the temperature by means of electrical heating elements, burners or injected chemicals which give off heat underground as a result of reaction. After the temperature of the portion to be ignited has reached a sufliciently high value, the hydrocarbons present in situ will combine with the oxidizing gas so that a combustion front is advanced through the formation. The combustion gases formed as a result of the combustion move in the direction of the production well. These gases contain, inter alia, water vapor which is formed partly by evaporation of the water present in the formation and partly by the combustion of hydrocarbons.

At a certain distance from the combustion front, the combustion gases are cooled to such an extent, by giving off heat to the formation, that the Water vapor in the combustion gases condenses, with the result that a condensation front is formed at some distance in front of the combustion front. This front heats the hydrocarbons at that locus and drives the latter in the direction of the production well.

Not all of the hydrocarbons are, however, displaced by the condensation front. A small portion (approximately of the pore volume) remains behind in the pores, which portion serves as fuel for the combustion front which moves towards the production well at some distance behind the condensation front. At those places through which a combustion front has been advanced by a combustion supporting mixture of oxygen: and inert gas, an almost entirely clean-burnt formation having a high temperature (for example, around l83( F.) remains behind. This process is known as dry combustion.

As a result of high compression costs, the quantity of oxidizing gas to be injected into the formation has a great influence on the economy of the processnIt has recently been suggested to minimize these compression costs by reducing the quantity of oxidizing gas necessary per unit of formation volume. This may be achieved by not allowing the temperature of the combustion front to exceed approximately 660 F., and not allowing it to fall below 400 F. (this being the lowest temperature at which oxidation of the residual hydrocarbons is possible at sufficiently high rate for the object envisaged).

In formations which are not too thick, the temperature of the combustion front may be controlled by limiting the quantity of oxidizing gas which is injected into the formation per unit of time. Preferably, however, the temperature is controlled by injecting Water into the formation. The air and water may be injected at rates such that only a portion of the injected quantity of water is evaporated at the combustion front. The water is introduced in such a quantity per unit of time that the temperature of the combustion front is equal to the temperature of saturated steam at the partial vapor pressure prevailing in the formation in the combustion front. This latter method (sometimes referred to as quenched combustion) may be used both in thick and thin formations.

It is known in the art to utilize the heat remaining behind in the formation by injecting cold water into the formation. The injected cold water evaporates in the formation before reaching the combustion front. This Withdraws heat from the rocks and causes the water to be transported in the form of water vapor to the combustion front together with the injected oxidizing gas. After having passed this front, water vapor is conveyed, together with the combustion gases, to the condensation front where it condenses. In this way the heat which otherwise would be lost to the process is utilized. In this process (sometimes called the wet combustion) the following zones in the formation may be distinguished (proceeding from the injection well to the production well):

(a) A clean-burnt cold zone containing oxidizing gas and water;

(b) An evaporation front in which the injected Water evaporates;

(c) A clean-burnt hot zone through which oxidizing gas charged with water vapor moves;

(d) A combustion front in which the oxidizing gas combines with the residual hydrocarbons, and a quantity of water vapor from the combustion reaction may be added to the water vapor already present;

(e) A zone in which residual hydrocarbons are present on the walls of the pores, and where combustion gases together with water vapor flow through the pores;

(f) A condensation front where the water vapor condenses and gives off heat to the formation and to the hydrocarbons present therein; and

(g) A zone in which the hydrocarbons are displaced to the production well.

As mentioned above, in the processes in which the temperature in the combustion front is controlled by the injection of water, water is injected in such quantities that the temperature of the combustion front is equal to the temperature of saturated steam at the partial vapor pressure prevailing in the formation in the combustion front or in such quantities that an evaporation front is maintained behind the combustion front. In the abovementioned temperature range of from 400 F. to 660 F., in which the combustion of the hydrocarbons takes place, the oxidizing gas substantially combines with the hydrogen atoms of the hydrocarbons present in the formation. Since more heat is liberated in the combustion of hydrogen per unit of oxygen than in the combustion of carbon per unit of oxygen, the quantity of oxygen supplied is in this way more profitably utilized than in the dry in situ combustion processes.

However, it has been found that the advantages of advancing a combustion front with air and water rather than air alone tend to be unattainable where the effects of gravity cause the combustion front to lay over within the reservoir. Where layover occurs, the water tends to flow down and the air tends to flow up to such an extent that, in the lower portion of the reservoir, the combustion is slow or extinguished, and in the upper portion of the reservoir, the combustion rates and temperatures are those of a dry combustion.

The tendency for a formation to so lay over is discussed in detail in a patent to Prats, U.S. Patent No. 3,208,516. This layover or gravity segregation is a phenomenon that occurs whenever a relatively low-density fluid is injected into a reservoir that contains a denser fluid and exhibits a significant amount of vertical permeability. Thus, in a forward-combustion drive, the injected air tends to rise above the oil and channel along the top of the formation. When water is injected with the air, the water tends to settle through the air and the oil and flow along the bottom of the reservoir.

SUMMARY OF THE INVENTION It is an object of this invention to reduce the layover tendency of the combustion front of a forward-combustion drive which is advanced by injection of a mixture of air and water into the formation.

It is a further object of this invention to produce a more nearly vertical combustion front in a formation in addition to ensuring that water reaches all of the regions being heated by the combustion front.

It is a still further object of this invention to prevent the segregation of water and air upstream of the combustion front within a formation even if layover occurs.

The objects of this invention are carried out by initiating the combustion of hydrocarbons within an underground formation contiguous to an injection well penetrating the formation so as to establishfa combustion front therearound. The combustion front is maintained and displaced towards a production well by injecting combustion supporting gas into the formation through the injection well. The combustion front is controlled at a temperature high enough to permit oxidation of "the residual hydrocarbons within the formation at a relatively high rate without generating excessive temperatures by injecting a quantity of water into the formation through the injection well cocurrently with the injection of the combustion supporting gas. A surface-active foaming material is mixed with the gas and water so as to move'the gas and water through the formation in the form of a foam. The foam keeps the water mixed with the gas and thus, even in reservoirs where segregation occurs due to the oil becoming thermally mobilized and settling below the incoming combustion supporting gas upstream of the combustion front, keeps the combustion from attaining the high temperature of dry combustion that would be attained if the water and gas were not converted into a foam.

BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 schematically shows a top plan view of a portion of a formation containing hydrocarbons, into which formation issue injection and productions wells from the s r ace of the earth;

DESCRIPTION OF THE PREFERRED .EMBODIMENT Referring now to the drawings, the numerals 1 and 2 therein designate injection and production wells, respectively, issuing into a formation desired to be produced. The wells 1 and 2 are illustrated schematically in FIG- URE 1 and are shown diagrammatically in FIGURES 2 to 4. in order that the temperatures graphically illustrated in the latter figures may be shown as a function of the distance between the wells land 2. Although the foregoing invention will be described with reference to particular processes for advancing a combustion front by injecting a mixture of air and water, the teachings of this invention may be carried out in any forward-combustion oil-recovery process of the type in which the combustion front is advanced by injecting a mixture of combustionsupporting gas and water.

The graphs in FIGURE 2 to 4 show the processes at the moment when a given volume of the formation (in all examples the same) has been traversed by a condensation front 3. The shading of areas in these figures is merely for the purpose of delineating the zones therein. For the sake of simplicity the graphs do not show the differences in temperature which result from the emission of heat to the formations lying above and below the formation to be treated, and to gases flowing through the formation to be treated. The temperature T, indicated in each of the graphs of FIGURES 2 to 4, designates the original ambient formation temperature.

DRY COMBUSTION PROCESS In the displacement of hydrocarbons by means of the prevlously known dry combustion process, as illustrated graph cally in FIGURE 2, oxidizing gas is injected into the in ection well 1 after a portion of the hydrocarbons III the formation has been raised to ignition temperature, the gas being injected in such a quantity that the temperature of the resulting combustion front 4 is maintained at, for example, approximately 1830 F., as illustrated by T in FIGURE 2. Except for losses of heat to neighboring formations and for losses of heat via the gas flowing through, the temperature of the burnt-out part of the formation, as designated by the zone 5, will be approximately 1830" F. At the same time the temperature over a zone 6 situated in front of the zone 5 is equal to the temperature of saturated steam at the local partial vapor pressure of water prevailing in the formation. The water present in the formation and the water formed by the combustion move in vapor form ahead of the combustion front 4 in the direction of the production well until, as a result of cooling, the temperature has dropped to the temperature T The water vapor condenses at the temperature T forming a condensation front 3 traveling in front of the combustion front 4, which condensation front drives the hydrocarbons ahead of it in the direction I of the production well 2, from which the hydrocarbons are produced by means well known in the art.

From the graph of FIGURE 2 it can be seen that the temperature of that part of the formation traversed by the combustion front 4 is substantially equal to the temperature of the combustion front 4, and that with the use of this dry" combustion process large quantities of heat remain behind in the formation unutilized. In this process, where the temperature of the combustion front is approximately 1830 F., approximately 10,000 kilo. cal., and approximately 4,500 kilo. cal. are obtained per kg. of residual hydrocarbon and per standard cubic meter of oxygen, respectively.

QUENCHED COMBUSTION PROCESS FIGURE 3 shows a quenched combustion process in which the temperature of the combustion front is controlled by the injection gf water. The temperature profile in the formation which is achieved by means of this process is shown diagrammatically in FIGURE 3. The quantity of injected water is such that the combustion temperature is substantially equal to the temperature of saturated steam at the partial vapor pressure of water prevailing in the formation at the combustion front. This temperature, designated as T typically lies between approximately 400 F. and approximately 660 F. Referring now to FIGURE 3, the temperature in the zone designated as 5" between the injection Well 1 and the combustion front designated as 4" is at T}, the original ambient formation temperature, as a result of the cooling by the injected water. The temperature of-the zone indicated as 6" located between the combustion front 4" and the condensation front 3 is equal to the temperature at the combustion fron-t, i.e., substantially equal to the temperature of saturated steam at the partial vapor pressure of water prevailing in the formation at the combustion front.

The injected water is converted into steam which is conveyed to the condensation front 3. The heat which remains in the formation behind the combustion front, is, in the process according to FIGURE 3, passed to the condensation front and is there utilized, resulting in a saving of oxidizing gas per unit of formation volume traversed by the condensation front.

In the process of the present invention utilizing gas and water injection, the heat liberated per kilogram of residual hydrocarbon and per standard cubic meter of oxygen is low relative to that of a dry combustion. Specifically, approximately 4,600 kilo. cal. and approximately 5,600 kilo. cal. are liberated per kg. of residual hydrocarbons and per standard cubic meter of oxygen, respectively. In this way, with the use of a combustion front temperature in the range of from approximately 400 F. to approximately 660 R, an important saving in the quantity of oxidizing gas required per unit of formation volume is achieved. This saving in quantity leads to a considerable reduction in the costs of compressing the oxidizing gas before the latter is injected into the formation since the injection pressure with the use of dry combustion process under similar conditions of formation pressure, injectivity, etc. is desirable.

WET COMBUSTION PROCESS FIGURE 4 shows the temperature profile in the formation which is obtained with the use of wet" combustion. In this case, the temperature T,' of the combustion front 4" is, for example, approximately 1830" F.

In the zone designated as 5" between the injection well 1 and the combustion front 4", there is an evaporation front 8. At this front, water injected via the well 1 evaporates in situ, after which the resulting vapor flows through the combustion front 4' to the condensation front 3. Thereby, heat which otherwise would remain behind in the zone 5" is transferred to the place where it can be utilized. The part of the zone 5 designated as 9 consists of clean-burnt formation having a temperature equal to that of the injected water (which temperature is substantially equal to the original ambient temperature T of the formation). The part of zone 5" designated as 10 has a temperature which is substantially equal to the combustion temperature T1.

' In the wet combustion process, at a combustion temperature of approximately 1830" F., the combustion of the residual hydrocarbons is complete and approximately 10,000 kilo. cal. are generated per kg. of residual hydrocarbon and approximately 4,500 kilo. cal. are obtained per standard cubic meter of oxygen. The quantity of heat liberated per unit of the formation volume traversed by the combustion front in the quenched combustion process in which gas and water are injected for the control of the temperature in the combustion front, is smaller than in the wet combustion. Accordingly, as can be seen from FIGURES 3 and 4, the distance between the condensation front and the combustion front will be smaller in the quenched combustion than in the wet combustion process. In the quenched combustion, smaller losses of heat occur in the transport of the steam from the place where the steam is generated (combustion front) to the place where it should be utilized (condensation front).

EXAMPLE In a given case observed over a period of 3 years, the average velocity ratio between condensation front and combustion front was 2.23 for the wet combustion and 1.53 under the same conditions for the quenched combustion process. In other words, when the condensation front has traversed a similar formation volume in both processes, as illustrated diagrammatically in FIGURES 3 and 4, the combustion front will have traversed 65% of this volume in the quenched combustion and 45% in the wet combustion, respectively. Since, compared with the wet combustion, only 40% of the quantity of oxidizing gas is required as necessary per unit "of formation volume traversed by the combustion front in the quenched combustion, an approximately corresponding savings in the compression costs of the oxidizing gas per unit of formation volume traversed by the condensation front is obtained with the quenched combustion as compared with the wet combustion.

APPLICATION OF INVENTION If the temperature of the combustion front is kept within the temperature range of 400 F. to 660 F. by means of water injection, a quantity of between 2.5 and 5 kg. of water should be injected per standard cubic meter of air. This includes the quantity of water which is normally necessary to ensure a water saturation in the part of the formation traversed by the advancing combustion front. Moreover, it is here assumed that the total quantity of injected water flows downstream from the injection well in the direction of the'combustion front. By a standard cubic meter is meant a volume of one cubic meter at a temperature of 68 F. and a pressure of 1 atmosphere.

It will be understood that, since the conditions in the various formations to be treated often widefl'y differ, no exact figures can be given concerning the required temperature at which the reaction in the combustion front will have to take place, and the quantity of water which, together with the air, should be injected into the formation.

In the preferred procedure, in which the reaction tem perature in the combustion front is kept between about 400 F. to 600 F. in order to obtain a selective combustion of the hydrogen atoms from the residual hydrocarbon, the preferred reaction temperature is, inter alia, largely dependent on the composition of the residue and the local pressure. For each formation to be treated, the temperature within the temperature range of 400 F. to 660 F. at which the hydrogen from the residual hydrocarbons is burnt selectively will therefore have to be determined experimentally by means of laboratory tests.

The same applies to the determination of the quantity of water to be injected. The most suitable water/air ratio, in the range of 2.5 to 5 kg. per standard cubic meter, should preferably be determined from laboratory tests, in which tests the formation conditions should be imitated as closely as possible.

The temperature of the combustion front moving through the formation may be measured in auxiliary wells drilled between the injection and production well( s). Moreover, an indication of the combustion temperature 7 may also be obtained by measuring the CO content of the combustion gases leaving the production well(s).

An excessively high temperature, or an excessively high percentage of CO gas in the combustion gases, shows the simultaneous combustion of carbon from the hydrocarbons in the combustion zone. The quantity of oxidizing gas supplied per unit of time should then be decreased. When water is injected, the quantity of injected water per unit of time may be increased. Specifically, the combustion temperature can be controlled by utilizing controlled injection of combustion supporting gas with water.

Thus, it can be seen from the foregoing that a forwardcombustion oil-recovery process has been described wherein a combustion front is advanced by injecting a mixture of air and water into the formation. However, as discussed hereinabove, the advantages gained by this process tend to become unattainable where the effects of gravity cause fluids to become segregated, with lighter fluid rising above heavier fluid, within the reservoir.

This tendency is overcome by mixing a quantity of surface-active foaming agent with the water and dispersing the air in the aqueous phase as a foam. The resultant foam is then displaced from near the point of injection to the combustion front at a rate suitable for advancing a combustion front.

Thus, the air and water are kept together by adding a foaming agent and dispersing the air in the aqueous phase as a foam. The foam remains intact as it is displaced through the reservoir, until it reaches the high-temperature zone near the combustion front. The air and the aqueous liquid containing the foaming agent are preferably injected simultaneously and continuously, alternatively, the air and aqueous liquid may be injected in alternate slugs.

The amount of foaming agent mixed with the air and water is not critical as long as it is sufficient to produce a relatively stable foam. In general, the concentration of the foaming agent is preferably approximately 0.1 percent by weight of water.

The ratio of the water of aqueous liquid to air may be varied to suit the desired conditions of wet or quenchecl combustion. If it is desired to evaporate the water before it reaches the combustion zone, the ratio of water to air is preferably kept below approximately two volumes of water per thousand standard volumes of air. In appropriate situations, higher ratios may be used where it is desirable to maintain a lower temperature at the combustion front.

Although the density of the foam is low relative to that of the water or oil in an oil reservoir, the use of the foam to advance the combustion front tends to reduce the extent of layover. Although the density of the foam is not much greater than that of air, its viscosity is materially greater than that of air. The tendency of foam to travel the most rapidly along the top of the reservoir is inhibited by the amount to which the radial extension of the foam-filled zone increases the amount of pressure that is required to maintain a given rate of flow through such an increasingly longer path. Thus, it can be seen that the use of the foam tends to produce a. more nearly vertical combustion front in addition to ensuring that water reaches all of the regions that are being heated by the combustion front.

In a reservoir containing a Water-rich layer below an oil-rich layer, an aqueous liquid containing a foaming agent may advantageously be injected into the waterrich layer. This may be accomplished prior to or during the injection of the mixture of air, water and foaming agent that forms the combustion-supporting foam. Such an increase in the concentration of foaming agent that is dispersed within the water-rich layer tends to increase the extent to which air is kept from channeling through the water-rich layer away from the oil.

In addition to reducing the layover of the combustion front, the use of foam to advance a combustion front tends to reduce the effects of thief zones and other inhomogeneities in the reservoir. For example, where the reservoir contains a gas cap or other layer through which fluids flow more readily than they flow through the liquidfilled portions of the reservoir, the use of foam materially reduces the extent to which fluids are channeled through the gas cap or other layer of high permeability.

EXAMPLE The process of this invention will be discussed with reference to an oil reservoir having a thickness of 50 feet and containing about 40 percent pore volume of oil having a viscosity of above 300 centipoises at the reservoir temperature. Preferably, at least two wells are opened into the full vertical interval of the reservoir on a spacing such that gas can be circulated between the wells at a rate of at least about 300 standard cubic feet per minute. The oil along the entire vertical interval encountered by an injection well, such as well I discussed hereinabove with reference to FIGURE 1. is ignited and sufiicient combustion-supporting gas is injected to initiate the ad vance of a combustion front towards a production well (such as well 2, also as in FIGURE 1 above). These operations are carried out by techniques well known to those skilled in the art.

After the combustion has been initiated, air, in an amount in the order of six million standard cubic feet, is injected to move the combustion front away from the injection well by a distance such as about 10 feet. The combustion front is then advanced by injecting an aqueous foam. This foam is preferably formed by dispersing about 0.2 percent by weight of a foaming agent in water and dispersing the air in the aqueous phase in a proportion of about two cubic feet of aqueous phase per one thousand standard cubic feet of air.

A suitable foaming agent is Adofoam, a liquid mixture of anionic surface-active agents, manufactured by Conoco Petrochemicals of Houston, Tex. Nonionic surfactants and mixtures of anionic-nonionic surfactants may also be used.

Where the spacing between the injection and production wells is 300 feet, the gas is preferably injected at a rate of about 300 standard cubic feet per minute and this advances the combustion front-from one well to the other in about 300 days. Since thecombustion front is kept relatively vertical, its advane is maintained at about one foot per day; and at time the first portion of the combustion front reaches the production well, the vertical sweep of the reservoir has'been substantially completed. Due to the presence of the water in the combustion= supporting fluid, the temperature of the combustion zone is maintained at from about 390 to 650 F.

In contrast, where a combustion front is advanced through a similar reservoir by injecting air alone, or by injecting air mixed with water that is free of a foaming agent, the advance of the upper portion of the combustion front is relatively rapid, due to a gravity layover as dis cussed in US. Patent No. 3,208,516. This causes a heat-= front breakthrough at an early stage in the process; and, due to the segnegation of the liquid and gas in the com= bustion-supporting fluid, the combustion is a dry com-= bustion having a relatively excessive temperature in the order of 1,000 F. or more.

In conclusion, it is noted that the present invention is not intended to be limited to the specific embodiment illustrated and described. For example, the invention may be practiced with a greater or lesser number of injection and production 'wells, the only requirement being that the wells be controlled in the manner exemplified by the foregoing detailed description. Further, the exact pattern of the production wells may be varied as long as wells to be successively produced are spaced from the injection well or wells by varying distances. Therefore, various changes in the details of the described process may be made, within the scope of the appended claims, without departing from the spirit of the invention.

I claim as, my invention:

1. A process for the secondary recovery of hydrocarbons from an underground formation penetrated by at least one injection well and at least one production well, comprising the steps of:

initiating combustion of hydrocarbons within the formation contiguous to the injection well to establish a combustion front therearound; maintaining said combustion front by injecting through said injecting well a combustion-supporting gas to maintain said combustion front and displace it through, the formation towards the production well; maintaining the combustion front at a temperature high enough to permit oxidation of the residual hydrocarbons within the formation at a relatively high rate by injecting a quantity of water into the formation through the injection well cocurrently with the injection of the combustion-supporting gas therethrough; and mixing the gas and water with a quantity of a surfaceactive material sufficient to produce a relatively stable foam that moves through said formation as a foam.

2. The process of claim 1 wherein the mixture of gas, Water and surface-active material is injected into the formation adjacent the combustion front in the form of a foam at a rate suitable for advancing said combustion front.

3. The process of claim 1 including the step of recovering hydrocarbons displaced from said formation through said production well.

4. The process of claim 1 wherein the step of maintaining the combustion front at a particular temperature includes continuously maintaining the combustion front at a temperature between 400 F. and 660 F. so as to selectively burn hydrocarbons within the formation to the exclusion of carbonaceous residues therein.

5. The process of claim 1 wherein the step of injecting a quantity of water includes injecting a quantity of water such that only a portion of said quantity of water is evaporated at thecombustion front.

6. The process of claim 1 wherein the step of injecting combustion-supporting gas into the formation includes injecting air; and

' the step of injecting a quantity of Water into the formation includes injecting a quantity of water of over 2.5 kilograms per standard cubic meter of air.

7. The process of claim 1 wherein the step of mixing a surface-active foaming material includes dispersing the gas in an aqueous liquid which includes the Water and sufiicient foaming material to form the relatively stable foam.

8. The process of claim 7 including the step of simultaneously and continuously injecting the gas and the aqueous liquid as a foam into the formation adjacent the combustion front.

9. The process of claim 7 including the step of injecting the gas and the aqueous liquid into the formation adjacent the combustion front in alternate slugs of gas and aqueous liquid.

10. The process of claim 1 wherein the step of mixing the gas and water with a surface-active material incliides mixing 0.1 percent by weight of surface-active material to gas and water. 11. A process for the secondary recovery of hydrocarbons from an underground formation containing a waterrich layer disposed below an oil-rich layer wherein 'the formation is penetrated by at least one injection well and at least one production well, the process comprising the steps of:

initiating combustion of hydrocarbons within the .for-

mation contiguous to the injection well to establish a combustion front therearound; maintaining said combustion front by injecting through said injection well a combustion-supporting gas to maintain said combustion front and displace it through the formation towards the production well; maintaining the combustion front at a temperature high enough to permit oxidation of the residual hydrocarbons within the formation at a relatively high rate by injecting .a quantity of water into the formation through the injection well cocurrently with the injection of the combustion-supporting gas therethrough; injecting a foaming agent only into the water-rich layer;

and mixing saidcombustion-supporting gas and Water with a quantity of a surface-active material sufficient to produce a relatively stable foam prior to injecting said mixture into said formation. 12. The process of claim 11 wherein the step of inject- 35 ing the foaming agent includes injecting the foaming agent simultaneously with the injection of gas and water into the formation.

13. The process of claim 11 wherein the step of injecting the foaming agent only into the water-rich layer is 40 completed prior to the injection of gas and water into the formation.

References Cited UNITED STATES PATENTS 3,150,715 9/1964 Dietz -1 16611 x 3,171,479 3/1965 Parrish et al l66-1l 3,182,721 5/1965 Hardy 166-1l 3,196,945 7/1965 Craig et a1. 16611 3,204,694 9/1965 Johnson et a1 16611 3,208,516 9/1965 Prats 166--11 X 3,208,519 9/1965 Moore 166--11 3,269,460 8/1966 Hardy et al 166-10 3,369,601 2/1968 Bond et a1. 1661l X STEPHEN J. NOVOSAD, Primary Examiner.

US. Cl. X.R. 166-274, 275, 309 

